A typical elevator includes an elevator car, a hoisting machine for moving the elevator car, at least one counter weight and traction means such as a rope, cable, chain, or belt. The traction means connect the elevator car and the at least one counter weight to each other. The traction means pass through a traction sheave which is connected to a drive shaft of the hoisting machine.
In a typical elevator having a counter weight the most power demanding situation is at the end of acceleration of a full elevator car into upward direction. When the acceleration ends the need of power is reduced. A similar situation occurs when empty elevator car is accelerated into downward direction. In other words this could be described as accelerating counterweight into upward direction. Thus, because of the counterweight power demanding accelerations can occur both directions.
Because of the weight difference between the elevator car and counter weight varies depending on the load of the elevator car, the actual use of energy also varies accordingly. The energy use depends on how much the hoisting machine needs to assist in accelerations and movement. Furthermore, it is known that sometimes, for example, in the moments of deceleration and moments when net weight is going downwards, energy is released. This released kinetic or potential energy may be transformed into electric power by using the hoisting machine as a generator. The generated electric power may be used for other purposes or fed back to the electric grid. In addition to the weights of the elevator car and compensating weight a person skilled in the art understands that the ropes used are typically heavy, particularly in high buildings where elevator cars are typically large and the ropes are long. These heavy ropes need to be taken into account when the energy demand of an elevator is estimated.
An elevator group comprises at least two elevators. A typical elevator group comprises adjacent elevators in the same building and in some cases the elevators are controlled by a destination control system. When using a destination control system a passenger makes a call, the destination control system registers the call and makes a call allocation. After the user has selected the desired floor, the system informs the passenger to which elevator car he should go, or which elevator car he should wait for. One advantage of the destination control system is that it reduces an average travel time because the elevator car makes fewer stops for individual passengers. A run of an elevator car starts when a brake of the hoisting machine is released and the elevator car starts moving. The run ends when the elevator car stops. The destination control system receives calls and provides information for an elevator group so that the group may be operated more efficiently, for example by optimizing waiting time of each passanger.
Elevator systems are complex and the energy consumption is high especially in high buildings. Thus, it is desirable to reduce energy consumption due to economic and environmental reasons.